In most cases you need to remove the retaining circlip and separate the bell and shaft from the rest of the motor....then undo any set screw(s) which hold the bell to the shaft... press the shaft out of the bell (NEVER hit it with a hammer!)... press a new shaft in... do up the set screw(s)... re-insert the shaft and press it through the stator so that the bell/shaft is in place... replace the circlip.